Recently, electronic equipment has been remarkably improved in performance, reduced in thickness, weight and size. For example, in the case of a portable telephone, it is equipped with camera function and increased in size of the display screen, remaining almost unchanged in overall shape and weight as compared with a conventional type. In order to make electronic equipment higher in performance and reduced in size, it is necessary to build as many electronic circuits as possible into a limited space. For efficiently using a limited space, besides the use of only a conventional printed circuit board, the use of both flexible board and printed circuit board is increasing. In the case of using both flexible board and printed circuit board, it is necessary to make the connection of these in reliable manner. Particularly, as the connecting pitch becomes lessened, short-circuit is liable to take place due to solder or the like between conductor patterns connected, and it is very important to prevent such short-circuit.
As an example of method for preventing such short-circuit, a method is disclosed in Japanese Laid-open Patent H8-32195. This method comprises the steps of laminating a circuit board with a first connection land group having a plurality of connection lands to a flexible board with a second connection land group arranged so as to be respectively opposed to the plurality of connection lands of the first connection land group, and connecting the connection lands to each other. Specifically, it is characterized in that an insulating layer having the following structure is formed and connected to a circuit board. That is, an insulating layer is formed so as to surround each of the plurality of connection lands of the first connection land group, and the insulating layer is structurally higher than the height of the first connection land from the surface of the circuit board. Thus, the connection land is structurally arranged inside the opening of the insulating layer. In such a structure, each of the second connection lands formed on the flexible board is fitted in the range surrounded by the insulating layer of the circuit board, and the second connection lands of the flexible board may be reliably connected in opposed positions to the first connection lands of the circuit board. Further, when the connection lands are connected to each other by soldering, the solder is restricted by the wall of the insulating layer and prevented from protruding sideways, thereby improving the reliability as disclosed.
Also, as another example, a method is disclosed in Japanese Laid-open Patent H9-82759. In this example, a projected electrode and solder layer are formed on a flexible board, and a spacer is provided around them. In this configuration, when the projected electrode of a flexible board is connected to the connecting terminal of a circuit board by means of the solder layer, the generation of short-circuit can be prevented because the spacer serves to prevent the solder from running.
In the first example, since the opening of the insulating layer formed on the circuit board is used as a guide for positioning the first connection land of the circuit board and the second connection land of the flexible board, positional deflection hardly takes place and it is possible to perform accurate soldering. However, when the connecting pitch is lessened, there arise the following problems. That is, if intended to make the connection with a fine pitch, it is necessary to make the second connection land formed on the flexible board less in thickness than the conventional configuration. If the second connection land of the flexible board is reduced in thickness, the insulating layer on the circuit board in the first example will be hard to effectively prevent the solder from running.
Also, in the second example, there is provided a spacer at the flexible board side, and the spacer serves to prevent the solder layer from running. However, a plating method is employed for forming a solder layer on the projected electrode of the flexible board. Accordingly, it becomes necessary to include an additional step to prevent other portions of the flexible board from being plated. Also, for mounting by reflow soldering together with other electronic parts to be mounted on the circuit board, it is desirable to use same solder material, but same solder material is not always available for plating, and it is sometimes difficult to execute such reflow soldering.